The present invention relates generally to subterranean operations involving displacement fluids and water-wetting surfaces within a subterranean formation. More particularly, the present invention relates to apparatus and methods for determining suitable displacement fluids for use in subterranean cementing operations, and methods of cementing that use such displacement fluids.
The drilling of well bores in subterranean formations commonly involves pumping a “drilling fluid” into a rotated drill string to which a drill bit is attached. The drilling fluid typically exits through openings in the drill bit, inter alia, to lubricate the bit and to carry cuttings up an annulus between the drill string and the well bore for disposal at the surface. One type of drilling fluid is an emulsion of substances that define a non-aqueous external phase and an aqueous internal phase. In this drilling fluid a non-aqueous “oleaginous” external phase (e.g., oil or synthetic polymers) may be used to inhibit swelling of water-sensitive drill cuttings (e.g., shale). Typical oil-based drilling fluids contain some amount of an internal aqueous phase. The emulsion often may be prepared by using an aqueous (water-based) internal phase comprising salts (e.g., calcium chloride). These oil- or synthetic-based drilling fluids also typically include chemical emulsifying agents that act to form oleaginous external phase emulsions, also known as “invert” emulsions. These chemical emulsifying agents also promote the oil-wetting of surfaces. This oil-wetted state promotes lubrication of the drill bit and further stabilization of formation materials.
After drilling is completed, a casing string commonly is cemented in the well bore as part of completing the well. One type of cementing operation includes placing a cement composition through the casing string and into the annulus to displace the drilling fluid from the annulus to the surface (however, flow in the opposite direction can occur in some operations, such as in reverse circulating or reverse cementing). A successful cementing operation also includes bonding the cement composition with the outer surface of the casing string and the inner surface of the well bore defining the annulus.
The bond formed between the cement composition and the outer surface of the casing string as well as the inner surface of the well bore may not be optimal if the casing string and well bore surfaces are not conducive to bonding with the cement composition. For example, the non-aqueous portion of the drilling fluid may coat the casing string and well bore surfaces. This may interfere with the bonding of the cement composition to the surfaces, because the aqueous cement composition generally will not bond readily with the non-aqueous substances that may coat the surfaces. If improper or incomplete bonding occurs at either of these surfaces, a thin region called a “micro-annulus” may be formed. Formation of a micro-annulus may lead to the loss of zonal isolation of the well bore, and undesirable fluid migration along the well bore casing string. Further, casing lifetime may be compromised if migrating fluids are corrosive.
Conventional attempts to solve this problem have involved displacement of the drilling fluid from the annular space between the formation and casing string, or between an inner casing and an outer casing strings so as to water-wet the formation and/or casing surfaces. Accordingly, it often may be desirable for the displacement fluid to invert the emulsion within the drilling fluid, while water-wetting the formation and/or casing surfaces.
A displacement fluid may be pumped ahead of the cement composition to create water-wet surfaces. Certain embodiments of such displacement fluids may cause a non-aqueous (hereafter “oleaginous”) external drilling fluid to invert, such that the aqueous internal phase becomes external, and the oleaginous phase becomes internal. Fluids that cause this inversion may be referred to as “inverter fluids,” and often may be suitable for use as displacement fluids. Examples of suitable inverter fluids include, inter alia, spacers and/or preflushes. Other nonlimiting examples of suitable inverter fluids include settable fluids and other compositions that comprise cementitious components such as hydraulic cements. Other nonlimiting examples of suitable inverter fluids are disclosed in, for example, U.S. Pat. Nos. 6,138,759, 6,524,384, 6,666,268, 6,668,929, and 6,716,282, the entire disclosures of which are incorporated herein by reference.
Conventional inverter fluids typically comprise an aqueous base fluid, viscosifying agents, and fluid loss control additives. Certain inverter fluids also may comprise, inter alia, weighting agents, surfactants, and salts. The weighting agents may be included in an inverter fluid, inter alia, to increase its density for well control, and to increase the buoyancy effect that the inverter fluid may impart to the drilling fluid and filter cake that may adhere to the walls of the well bore. Viscosifying agents may stabilize the suspension of particles within the inverter fluid, and may control fluid loss from the inverter fluid. The presence of a surfactant in the inverter fluid may enhance the chemical compatibility of the inverter fluid with other fluids (e.g., the drilling fluid, and/or a cement composition that subsequently may be placed within the formation) and may water-wet downhole surfaces, thereby improving bonding of the cement composition to surfaces in the formation, and may facilitate improved removal of well bore solids. A salt may be included in the inverter fluid, inter alia, for formation protection, improved compatibility among fluids in the formation, and to desirably affect wettability.
Inverter fluids also may be used to displace oleaginous-external/aqueous-internal fluids from cased hole or open hole well bores in operations other than cementing. One example involves replacement of these inverter fluids with a completion fluid (e.g., a solution of calcium chloride or bromide). This operation may be conducted to clean the well bore for further operations, such as perforation of the casing or, in the case of an open hole, the onset of production of the well. In this case, the inverter fluid may serve to displace the previous fluid and leave the formation surfaces in a water-wet state.
The use of inverter fluids in cementing and other subterranean operations often may be problematic, because of, inter alia, difficulties in identifying a specific inverter fluid composition that may desirably invert a particular drilling fluid composition in a manner so as to water-wet the annulus to a desired degree. Conventional attempts to identify specific inverter fluid compositions that may desirably invert a particular drilling fluid composition in a desired manner, at the temperature and pressure to which both fluids may be exposed in a subterranean formation, often have involved a multi-step process that may fail to identify incompatibilities between components of the fluids at the anticipated subterranean conditions. Commonly, a proposed inverter fluid composition has been pre-conditioned to the anticipated temperature and pressure using a high-pressure, high-temperature apparatus, then cooled, de-pressurized, and removed from the first apparatus, and placed in a testing apparatus at atmospheric pressure and only slightly elevated temperature, along with a sample of the drilling fluid that is to be inverted. This method is problematic because it may mask certain changes or conditions (e.g., cloud point changes, solubility changes, and the like) that may result in an incompatibility between the fluids and/or that may indicate that the proposed inverter fluid composition will not invert a particular drilling fluid composition in a desired manner at the desired temperature and pressure.